feat(adapter/llm): implement AnthropicAdapter

Implements AnthropicAdapter using the anthropic SDK.
- Reads ANTHROPIC_API_KEY from env; raises ValueError if missing
- resolve_model() looks up capability_map in team.yaml config,
  falls back to "capable" tier then hard-coded claude-sonnet-4-6
- complete() supports system_prompt, max_tokens (default 4096),
  and temperature (default 0) via the context dict
- Adds PyGithub to requirements.txt (needed by GitHubAdapter)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

requirements.txt

@@ -10,6 +10,9 @@ pyyaml
 # Environment variable management
 python-dotenv
 
+# GitHub VCS adapter
+PyGithub
+
 # --- stdlib-only (no pip install needed) ---
 # sqlite3   — blackboard persistence
 # dataclasses — task_brief schema